A variety of different ultrasonic signal detectors have been developed in the past in order to monitor a broad range of ambient signals in the ultrasonic range and to convert those signals to audible frequencies. The desirability of ultrasonic detectors has recently increased due to the recognition that ultrasonic detectors may readily be implemented as leak detectors to detect ultrasonic signals which, for example, are created by the escape of pressurized gases through small openings. This is useful, for example, in detecting leakage from pressurized telephone trunk lines, pipelines and in detecting air flow paths, for example, through installation of houses and commercial buildings and through automobile doors and panels.
In the last few years, two types of ultrasonic detectors were prevalent. A first type employs a crystal system to mechanically couple an ultrasonic signal to a local oscillator in order to convert the frequency of the input ultrasonic signal to a resultant signal that has a frequency within the audible range. While being relatively inexpensive, crystal based systems have exhibited limited performance and have significant problems of sensitivity. These crystal based systems are susceptible to noise and have problems with signal-to-noise ratio. In addition, crystal base systems are susceptible to mechanical vibrations and to temperature changes which can affect their sensitivity and yield false readings. Further, crystal based systems often and undesirably respond to infra-sonic and sonic signals that modulate the system so that again faulty readings occur. These crystal based systems also often have a very limited frequency range for target signals unless there is an ability to adjust the frequency of the local oscillator within the system.
A second system commonly used employs signal mixers that heterodyne a local oscillator with the input signal to generate a composite signal within an audible range. The composite signal is then output through a transducer, such as a pair of headphones, and the intensity of the composite signal, presumably reflective of the intensity of the detected ultrasonic signal, may be displayed by a suitable meter. Heterodyning was done in the past in a number of ways, where the mixer, the most important element, was made either from a diode-ring, an electromechanical device or any non-linear device like a vacuum tube or transistor. Because each of the necessary circuit elements in these translators is discrete, matching components or trimming during production was inevitable. In addition, the noise in the translated audio was quite high due to oscillator energy leakage in the heterodyned signal due to mixer inefficiency. A typical ultrasonic leak detector using an older mixer for translation produces a fair amount of hissing sound even when there is no signal present. This is due to the leakage of high frequency components from the multiplication of the two signals, the oscillator and the incoming sound. Non-linear mixers generate a multitude of mixed products, some of which are in the base band, thus contributing to the noise floor of the system.
Again, these heterodyne systems are susceptible to noise and have a problem with signal-to-noise ratio. Moreover, ultrasonic detectors using the heterodyne circuitry have a problem with signal-to-noise ratio and have a limited frequency range unless the oscillator for each can be adjusted. While these systems do not exhibit problems due to sonic or mechanical vibrations, they are nonetheless susceptible to temperature changes that can yield faulty readings. Further, systems that employ the heterodyne technology require multi-offset settings and are thus difficult to adjust and maintain over an extended period of use.
A much improved ultrasonic detector is described in my U.S. Pat. No. 5,103,675 issued Apr. 14, 1992. This ultrasonic detector offers superior performance to those discussed above. In its detailed circuitry, the ultrasonic signal detector described in the '675 Patent detects ultrasonic signals by a transducer and converts these signals into an electrical transducer signal that is then filtered to remove undesired frequency components so that a resulting input signal results. Processing circuitry is then responsive to the input signal for producing an intermediate signal that has an intermediate signal frequency scaled from the input signal frequency into an audible frequency. The intermediate signal is then amplified proportionally to the input signal amplitude to produce a detector output signal which is then displayed. In the '675 Patent, then, as described in the detailed embodiments, a square-wave signal is produced that has a frequency corresponding to the frequency of the input signal, but at a constant amplitude. This square-wave signal is then scaled to an audible frequency and is then integrated to produce a triangular-wave pulse having this scaled frequency. This triangular-wave pulse is amplified proportionally to the amplitude of the input signal to create a replica of the input signals at the lower frequency.
Despite the improvements of the invention described in the '675 Patent, there remains a need for low cost ultrasonic detectors that are reduced in size and which are inexpensive to use. Indeed, many members of the industry are more comfortable with the tones produced by a heterodyne circuit, including the background noise, since they are more familiar with this technology. Here, the noise is often introduced since the standard heterodyne circuit simply applies a local oscillator frequency along with the input signal to the base of a transistor to generate the composite signal in the audible range. Thus, any noise in the received signal or created by oscillator leakage is passed through to the audible output. Furthermore, where a signal strength indicator is used, the signal strength is usually integrated over the entire spectrum of frequencies as an average of the signal processed by the detector rather than the true amplitude of the detected signal.
Recently advances in the telecommunications industry have produced microcircuits designed for cellular communications which operate in the realm of 100 MHz. These micro-circuitry chips provide very sensitive heterodyne circuits employing a Gilbert Cell mixer to blend a very high frequency local signal against a received very high frequency signal, typically in the range of 100 MHz to 140 MHz. The use of Gilbert Cell mixer technology has not heretofore recognized as appropriate for ultrasonic detectors due to the relatively low frequencies encountered in the ultrasonic environment. Moreover, it has been thought that the technology embodied in the very high frequency telecommunications circuitry could not, in fact, be employed at the lower frequencies of ultrasonic signals. I have surprisingly found that, with the circuit design of the present invention, this technology can, indeed, be employed to produce a very low cost, compact ultrasonic detector which exhibits superior performance.